1. Technical Field
The invention is related to microwave plasma reactors for processing semiconductor wafers to perform etch or chemical vapor deposition or similar processes on the wafer.
2. Background Art
A plasma reactor may be employed to perform various processes on a semiconductor wafer in microelectronic fabrication. The wafer is placed inside a vacuum chamber of the reactor and reactant gases are introduced into the chamber. The gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Depending upon the composition of the gases from which the plasma is formed, the plasma may be employed to etch a particular thin film from the wafer or may be employed to deposit a thin film layer onto the wafer. In order to maximize processing throughput, it is desirable that the plasma have a high ion density. High ion density provides, for example, a high etch rate on the wafer, so that less time is required to perform a given etch process, thereby increasing throughput. To provide a high density plasma, a large amount of electromagnetic power must be applied to the plasma, which may increase ion energy in addition to increasing plasma ion density. The problem is that unduly high ion energy tends to damage microelectronic devices on the wafer, thus decreasing device yield. This problem is avoided in the prior art by employing reactors in which the ion density and the ion energy may be controlled independently.
One conventional type of plasma reactor in which ion density and ion energy are separately controllable is an inductively coupled plasma reactor in which ion density is determined by the amount of radio frequency (RF) power applied to an inductive coil antenna surrounding the reactor chamber while ion energy is separately determined by the amount of RF power applied to a pedestal underlying the wafer. A disadvantage of such an inductively coupled plasma reactor is that such RF powered coil antennas tend to be inefficient in maintaining plasmas uniformly across the chamber. Another disadvantage is that the power loss through the coil is a very sensitive function of the Q-factor of the coil, and process repeatability is relatively poor.
Another conventional type of plasma reactor with independent control of ion energy and ion density employs electron cyclotron resonance (ECR) to produce microwaves to ignite and maintain the plasma in the chamber. While ECR reactors are more efficient in producing a plasma because of the relatively high frequency of the microwaves (compared with the RF frequencies employed in inductively coupled plasma reactors), ECR plasma reactors require delicate control of the location and magnitude of external magnetic fields. Moreover, ECR works well only at very low chamber pressures, since at higher pressures electron collisions with neutral species suppress the required resonance.
It is therefore an object of the invention to provide a plasma reactor which enjoys the advantages of both the inductively coupled plasma reactor and the ECR plasma reactor without encountering the problems of either.
In conventional plasma reactors, reactant gases are combined in a manifold and then delivered to the reactor chamber via a single gas feed line to a gas distribution plate in the ceiling of the chamber. The gas distribution plate has many gas flow orifices in an area overlying the wafer, and the placement of these orifices determines whether the gas flow is uniform across the wafer surface. Since the orifices are permanent features of the gas distribution plate, their precise placement for optimum uniformity of gas flow must be learned through expensive and time-consuming trial-and-error experiments. Typically, gas distribution patterns are tailored for a specific application (e.g., chemical vapor deposition of a particular material to a particular thickness over a particular layer for a particular size wafer), so that for each such application the trial-and-error method of optimizing placement of the gas orifices in the gas distribution plate must be repeated.
It is therefore a further object of the present invention to eliminate the need for customizing each gas distribution plate with such an inefficient trial-and-error method.